In recent years, low voltage and single power supply have been required of a flash memory as a nonvolatile semiconductor memory device and, therefore, there is a demand for a semiconductor integrated circuit which performs boosting or negative boosting on chip to supply voltages required for writing, erasing, and the like.
FIG. 26 is a block diagram illustrating a conventional semiconductor integrated circuit (hereinafter referred to as a semiconductor IC).
With reference to FIG. 26, the semiconductor IC comprises a booster 201 and a regulator 202. The booster 201 boosts a power supply voltage V.sub.DD applied to this semiconductor IC, to a predetermined voltage V.sub.PP. The regulator 202 is supplies with the boosted voltage V.sub.PP, and regulates the boosted voltage V.sub.PP to output an output voltage Vo. The regulator 202 comprises a reference voltage generator 203, a differential amplifier 204, an output circuit 205, and a voltage divider 206.
The reference voltage generator 203 is supplied with the boosted voltage V.sub.PP from the booster 201, and generates a reference voltage Vref. The reference voltage generator 203 can change the reference voltage Vref to plural voltages. The differential amplifier 204 is supplied with the output voltage V.sub.PP from the booster 201 through a power input terminal and, further, it is supplied with the reference voltage Vref generated by the reference voltage generator 203 and a divided voltage Vd (described later) from the voltage divider 206. The differential amplifier performs differential amplification on the basis of the voltage V.sub.PP, and outputs a voltage Va so obtained. The output circuit 205 includes a P type MOS transistor having a gate connected to the output terminal of the differential amplifier 204, a source connected to the output terminal of the booster 201, and a drain connected to the input terminal of the voltage divider 206. The output circuit 205 outputs, as an output voltage Vo from the regulator 202, a voltage obtained by regulating the output voltage V.sub.PP from the booster 201 on the basis of the output voltage Va from the differential amplifier 204. The voltage divider 206 is supplied with the output voltage Vo from the output circuit 205, and outputs a divided voltage Vd obtained by dividing the output voltage Vo.
Hereinafter, the operation of the conventional semiconductor IC so constructed will be described.
The booster 201 generates a boosted voltage V.sub.PP which is higher than the power supply voltage V.sub.DD from the power supply voltage V.sub.DD, and outputs this voltage V.sub.PP to the regulator 202. The regulator 202 outputs a predetermined constant voltage Vo obtained by decreasing the boosted voltage V.sub.PP, from its output terminal.
In the regulator 202, the reference voltage generator 203 is supplied with the boosted voltage V.sub.PP, generates a predetermined reference voltage Vref, and outputs it. Accordingly, the reference voltage Vref has a value in a range from the boosted voltage V.sub.PP to a ground voltage V.sub.SS. The voltage divider 206 outputs a divided voltage Vd which is obtained by dividing the output voltage Vo from the regulator 202, according to a predetermined voltage ratio r (r.gtoreq.1), so as to satisfy the relationship VO/Vd=r. The output voltage Vd from the voltage divider 206 is compared with the reference voltage Vref by the differential amplifier 204, and the P type MOS transistor M10 in the output circuit 205 is controlled by the output voltage Va from the differential amplifier 204, resulting in Vd=Vref. In this way, the regulator 202 is able to generate an output voltage Vo which is maintained at a constant voltage, i.e., Vo=r.multidot.Vref, from the boosted voltage V.sub.PP which is not always stable.
Further, the regulator 202 is required to provide different output voltages Vo for different modes of the nonvolatile semiconductor memory device, such as writing, erasing, etc. In this case, supply of voltages suited for different modes is realized by changing the reference voltage Vref for each mode.
Further, although a power supply circuit performing positive boosting has been described above, a conventional semiconductor IC for generating a negative voltage is similar to the above-described circuit. In this case, in the semiconductor IC shown in FIG. 26, the booster 201 is replaced with a negative booster, and the P type MOS transistor M10 in the output circuit 205 is replaced with an N type MOS transistor, whereby a semiconductor IC which is able to output a constant negative voltage with reference to a negative reference voltage is obtained.
In the conventional semiconductor IC, however, since the regulator 202 operates with the output voltage V.sub.PP from the booster 201, a great load is applied to the booster 201. Usually, the booster 201 is a charge pump circuit, and the output current vs. output voltage characteristics are as shown in FIG. 27.
FIG. 27 is a graph showing the output current I.sub.PP vs. output voltage V.sub.PP characteristics of the charge pump circuit. The abscissa indicates the output current I.sub.PP, and the ordinate indicates the output voltage V.sub.PP. As seen from the graph of FIG. 27, the output voltage V.sub.PP from the booster 201 decreases with an increase in the output current I.sub.PP. Accordingly, when the load on the booster 201 increases, the output current I.sub.PP increases, resulting in difficulty in obtaining a predetermined output voltage V.sub.PP. Especially, in order to secure a boosted voltage V.sub.PP higher than a predetermined level to achieve a reduced power supply voltage, the number of stages of the charge pump circuit must be increased, but this causes further increase in the reduction radio of the output voltage V.sub.PP to the output current I.sub.PP. Therefore, the capacitance in the booster 201 must be increased to maintain the output voltage V.sub.PP from the booster 201 at a predetermined level, resulting in an increase in the area of the booster 201.
Likewise, also in the conventional semiconductor IC for generating a negative voltage, since the regulator operates with the output voltage from the negative booster, a great load is applied to the negative booster. Therefore, like the booster described above, the output current from the negative booster increases, and it becomes difficult for the negative booster to secure a predetermined output voltage. Also in this case, in order to secure the output voltage, the area of the negative booster must be increased to increase the capacitance in the negative booster.